1. Field of the Invention
The present invention relates to a process and a system for detecting decrease in tire air-pressure and a tire deflation judging program. More particularly, it relates to a process and system for detecting decrease in tire air-pressure and a tire deflation judging program with which it is possible to detect decreases in air-pressure of tires with good accuracy even in cases where a drive of a vehicle is repeated in a short duration of time (here, the “drive” refers to a period from the start of the engine to the turn-off thereof).
2. Description of the Related Art
A system for detecting decrease in tire air-pressure employs a theory that a wheel velocity or a rotational angular velocity of a tire with a decreased air-pressure is increased when compared to a tire with a normal internal pressure due to a decrease in outer diameter (dynamic load radius of the tire) from that of a tire with a normal internal pressure. In a method for detecting decrease in internal pressure on the basis of a relative difference in wheel velocity, DEL={(V1+V4)/2−(V2+V3)/2}/{(V1+V2+V3+V4)/4}.times.100(%) is employed as a judgment value (reference should be made to Patent Document 1). Here, V1 to V4 denote rotational angular velocities of a front left tire, a front right tire, a rear left tire and a rear right tire, respectively.
By the way, the foregoing judgment value determined on the basis of a relative difference in wheel velocity of the tires somewhat varies depending on the driving conditions of the vehicle.
For this reason, in the conventional system for detecting decrease in tire air-pressure, an average value obtained by taking the average of relative difference values that are obtained during a predetermined consecutive period of time, for example, 5 seconds is employed as a judgment value and the average of a prescribed number (e.g. 6)of the judgment values is calculated so that a detection of a decrease in tire air-pressure is accomplished by judging whether the average value exceeds a predetermined threshold value or not.
However, when the foregoing detection of decrease in tire air-pressure is applied to a case where a short distance travel of a vehicle (for example, postal truck) that completes within 30 seconds or less is repeated, since there is no chance to calculate an average value even when the judgment value exceeds the threshold value, it is impossible to detect the decrease of air-pressure even if a decrease in air-pressure occurs to a tire.
For this reason, it is arranged such that obtained judgment values are stored in a nonvolatile memory so as to enable calculation of the average value even when a drive of a vehicle is discontinued by using judgment values obtained during the drive before it is discontinued together with judgment values obtained during the subsequent drive.
[Patent document 1] Japanese Unexamined Patent Publication No. S63-305011
[Patent document 2] Japanese Unexamined Patent Publication No. 2002-19435
Conventionally, a memory having a prescribed number of data areas required for taking an average is used as the nonvolatile memory for the foregoing average calculations. This leads to the problem that as the prescribed number of data areas increases, a redundant capacity is required for the nonvolatile memory, causing the size of the memory construction to be larger than is necessary.